The invention relates to a measuring chamber for photo-acoustical sensors for the continuous measurement of radiation-absorbing substances, in particular of radiation-absorbing particles, in gaseous samples comprising at least one entry and at least one exit for the samples, a tube section with microphone that allows for the flow-through of the sample in longitudinal direction and with at least one entry and one exit point for the laser beam aligned with the tube section, and whereby said entry and exit points are arranged at a distance relative to the measuring tube respectively by way of at least one chamber with a cross-sectional area that is expanded relative to the tube section.
Photo-acoustics is a very sensitive measuring technique in order to determine, for example, trace gas or aerosol concentrations in a carrier gas. In the-photo-acoustical measuring process a solid, liquid or gaseous sample containing at least one (possibly frequency-selective) radiation-absorbing substance is irradiated with intensity modulated electromagnetic radiation; frequently this is visible or infrared light. Since the substance absorbs the radiation, the substance becomes heated and the heat is given off to the environment during the pauses with low radiative intensity. This results in a periodic heating and cooling of the irradiated volume, which in turn leads to periodic pressure fluctuations propagating in the form of sound waves that can be detected with the use of sensitive microphones. The method is depicted schematically in FIG. 1.
Resonant cells are used to increase the sensitivity by harmonizing the period and/or the frequency of the modulated irradiation with the characteristic frequency of the measuring cell. For a time-resolved measurement of substances in gases the carrier gas must flow through the cell. A simple cell with longitudinal resonance is, for example, described by Krämer and Niessner in the German utility model no. 200 17 795.8 and by Beck, Niessner and Haisch in Anal. Bioanal. Chem. 375 (2003), p. 1136 et seq. This cell is represented schematically in FIG. 2. It is comprised of a tube R the length of which determines the resonance frequency and the diameter of which is considerably smaller than its length. The areas AN with expanded diameter on both ends of the resonance tube R are referred to a “notch” filters. The change in diameter can be viewed as a “fixed end” for the acoustic pressure wave and therefore generates a node of the pressure wave and (A pressure node corresponds to a maximum of the particle velocity). The length of the entire measuring cell amounts to approximately one wavelength lambda of the sound wave, with the following equation indicating the connection of the wavelength lambda relative to the resonance frequency:Lambda=sound velocity/resonance frequency.
A disadvantage of the shown measuring cell is its susceptibility to soiling of the windows through which the intermittent radiation L enters and exits. This is in particular a grave problem if the photo-acoustic cell is used for measuring aerosols, e.g. soot particles from combustion engines or in general substances from the environment. Flow calculations have shown that, for example, the gas entering on the left of the resonant cell forms vortexes causing a flow toward the window already in the left “notch filter”, whereby some particles of the measured aerosol become deposited at the window location resulting in a parasitic effect. After the measured gas passes through the resonant cell, it flows directly toward the opposite window, whereby once again some particles of the measured aerosol are deposited also causing a parasitic effect. The parasitic effect results from the fact that the deposits on the windows also absorb radiation and generate sound waves that superimpose on the measuring signal in the form of interference. An exact measurement, in particular for low concentrations of the measured aerosol, is thereby prevented. The result of a finite element calculation of the flow in this cell is shown in FIG. 3; whereby direction and velocity of the flow are characterized in the figure by the direction and length of vectors.
The usual method for keeping the optical windows in a flow-through apparatus clean, such as in opacimeters provides to rinse the windows with a flow of particle-free air thereby preventing the contamination of the windows by particles. This method is not usable in a photoacoustic measuring cell according to FIG. 2 for acoustical reasons: the rinsing air flow causes a “whistling” sound meaning that an essentially higher parasitic signal is superimposed over the desired measured signal.
It was the object of the present invention to provide a measuring cell that will reduce the soiling of the windows at the entry points of the radiation into the cell and that will slow down the deposition of the particles of the measured aerosol thereon in such a way that operation of the measuring cell is possible with a high level of sensitivity over long periods of time, while avoiding the above referred to disadvantages